There is an ever increasing need for providing reliable interconnections between plastic and metal pipe which are effective over a wide temperature range to maintain good sealing relation between the two. For example, in the supplying of natural gas, it is customary, at the present time, that the main lines located in the street are constructed of steel whereas feeder or service lines to houses and adjacent buildings are most frequently constructed of a plastic, such as polyethylene. Also, it is required that steel pipe be used above ground in making connections to gas regulators and meters, which frequently interconnect the underground plastic pipe to the main via so-called "risers". Still further, steel line extensions commonly join to plastic mains. In each of these situations, it is necessary to provide a suitable transition fitting that will accommodate both the plastic and steel pipes. The primary obstacle that a successful transition fitting must overcome is the considerable difference in temperature coefficients of expansion of the two materials. Although actual expansion rates will depend upon a particular plastic, in general it can be shown that plastics used in manufacturing pipe have an expansion coefficient of four to five times that of steel. Therefore, where the ambient temperature range is extensive a satisfactory transition fitting must accommodate for the "working" of the pipes and interconnection parts resulting from the unequal expansion and contractions, in order to preserve the interconnection between the two, as well as to prevent breakage or damage. Most known past transition fittings for this purpose have been compression fittings and when plastic is compressed there is a tendency for it to cold-flow with the concomitant reduction in holding strength and the increased possibility of leakage. Other transition fitting designs hold the plastic in contact with the steel firmly and do not allow the plastic to move thereby preventing the plastic pipe from relieving thermal or other stresses.